This invention relates to improved drying processes. More specifically, it relates to improved devolatilizing and drying techniques utilizing microwave heating.
In conventional processes for making polymers, it is necessary to remove solvents and/or water from the polymer. For example, in the preparation of butyl rubber, a multiolefin and an isoolefin are reacted at a temperature of about -40.degree. F. to about -160.degree. F. in the presence of a Friedel-Crafts catalyst such as aluminum chloride. The catalyst is ordinarily aluminum chloride in methyl chloride and the mixture of monomers, e.g., isoprene and isobutylene, contacts the catalytic solution in a tubular type of reactor at the low temperature conditions to form a slurry of butyl rubber particles in a diluent which may also be methyl chloride. The slurry is then introduced into boiling water to flash off the methyl chloride and form a slurry of butyl rubber, generally in the form of crumbs, in the water. The rubber crumb is then removed from the water slurry and dewatered on a vibrating screen or Oliver type rotary filter to about 30 to about 60% water by weight. It is usually further mechanically dewatered, e.g., Anderson Expeller or dewatering extruder, to a water content of about 6 to about 20 wt. %.
An Anderson Expeller is a continuous mechanical screw press employing discontinuous worms on a shaft separated at intervals by collars and breaker lugs and operating within and through a drainage barrel made up of bars separated by thin spaces. The screw moves the material from the hopper, through the drainage barrel. As the solids move through the barrel under pressure, the liquid is pressed from the solids and permitted to escape through spaces between the bars that make up the barrel.
Thereafter, the remaining water is removed by heating. For example, the crumb is fed into a heated devolatilizing extruder in which mechanical energy and external heat provide the heat necessary to vaporize the water. The devolatilizing extruder may be equipped with a die face pelletizer. The pellets so formed are substantially free, i.e., 0.05 to 0.5 wt. %, of water. They may be formed under water or dropped into water to cool to prevent agglomeration of the pellets. The surface water is removed by blowing with air, e.g., in a drying tunnel. The pellets are then cooled and baled, under pressure, in the manner described in U.S. Pat. No. 3,264,387 which is incorporated herein by reference.
The baling process is generally performed at a temperature of about 140.degree. to about 250.degree. F. and a pressure of about 800 to about 3500 psi: the heat and pressure being maintained for about 5 to about 60 seconds. The resulting compacted mass has a density of about 40 to about 54 pounds per cubic foot.
Similar processes incorporating water separation and drying steps are inherent in other polymer processes. For example, styrene butadiene (GR-S) rubber is prepared as a rubber latex which is coagulated by the addition of acid or salts. The coagulated product is washed, filtered on an Oliver rotary vacuum drum filter, from which it is removed, having a water content of about 30 wt. % to about 50 wt. % and dried for about 20 minutes to 2 hrs. at a maximum of 82.degree. C. to reduce the volatile matter content (i.e., water) to about 0.5%.
It is readily evident that large space requirements and equipment costs are needed in these conventional drying operations. Additionally, in certain processes, the heating steps are either inadequate or detrimental.
For example, the shearing action and high temperature in the devolatilizing extruder, i.e., 375.degree. -500.degree. F., results in polymers (e.g., PVC, butyl rubber) having molecular weight distributions skewed toward the low end. Consequently, it is only with great difficulty that, in preparing butyl rubber, can a polymer be prepared which has a weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) greater than 5.0.
The halogenated butyl rubbers, especially brominated butyl rubber, or example, decompose at the drying temperatures used in the devolatilizer extruder. The result is the release of lacrimatory gases, e.g., HBr, and severe corrosion of equipment.
When polyisobutylene has been prepared and dried in a conventional manner, baled in the manner described in U.S. Pat. No. 3,264,387 and stored for several weeks, the bales become relatively transparent. Occluded moisture becomes visible as a large white "baseball" in the center of the bale.
Through the polymer water content is only about 0.1 to about 0.8 wt. %, and may not affect product quality, the appearance of the bale affects customer acceptance. It is desirable therefore to remove this remaining water to give a moisture-free product. Conventional drying techniques are inadequate to further reduce the water content of the polymer.
Various polymer processing techniques have been developed utilizing electronic heating. For example, metal particles have been dispersed in a vulcanizable rubber and the mix cured by induction heating at a frequency of about 1 MHZ, e.g., see U.S. Pat. No. 3,249,658. As the name implies, induction heating operates by inducing a current in a conductor, i.e., metal filings; the heat effect depends on the eddy currents induced in the material and the heating of the rubber is by conduction from the metal filings.
Dielectric heating has been used to heat non-conductors having polar molecules. For example, polyvinyl chloride may be pressed into molding "pre-forms" and heated by dielectric heating prior to introduction into a compression mold. This heating technique relies on the polarity of the molecule to induce a heating effect. The material to be heated is placed between two plates which form a capacitance in an electronic circuit. The polarity of the plates is rapidly reversed at a frequency in the range of about 1 to about 150 MHZ. Heating is caused by the rapid vibration of the polar molecules attempting to align themselves with the constantly changing field.
More recently the partial curing of natural rubber or synthetic elastomers has been accomplished by passing the material through the center of a helical metal wave guide which is connected to a microwave generator running at about 300 to about 30,000 MHZ, e.g., British Pat. No. 1.065,971. Curing is completed by passing the material through a conventional heater.
Microwave heating, like conventional dielectric heating, is based on the principle that electromagnetic waves interact with a dielectric material, some of the energy associated with these waves being stored and some being dissipated. The heating effect is a function of the dissipated energy (dielectric loss). The dielectric loss is caused by the frictional drag associated with permanent or induced dipole orientation in the alternating electric field. Generally, polymers show an increase in dielectric loss with an increase in frequency of the radiation. Some polymers, however, e.g., PVC, actually show a decrease in dielectric loss at the higher frequencies.
Though all polymer molecules exhibit some polarity, with few exceptions, the synthetic elastomers are essentially nonpolar and hence, have a low dielectric loss. Heating of these materials is usually accomplished by the inclusion of polar materials such as fillers, i.e., carbon black.
In the conveying of polymers around the polymer plant, transfer lines are used. These transfer lines consist of pipe through which air is blown at sufficient velocity to move the polymer. These pneumatic conveyors may be pressure operated, suction systems, or a combination of the two. The design and operation of these pneumatic conveyors are well known to the art. The transfer line concept has been used in other industries to convey particulate matter, e.g., the removal of lime from railway cars.